Interaction is a kind of action that occur as two or more objects have an effect upon one another. The idea of a two-way effect is essential in the concept of interaction, as opposed to a one-way causal effect. A closely related term is interconnectivity, which deals with the interactions of interactions within systems: combinations of many simple interactions can lead to surprising emergent phenomena. Interaction has different tailored meanings in various sciences.[citation needed] Changes can also involve interaction.

Communication of any sort, for example two or more people talking to each other, or communication among groups, organizations, nations or states: trade, migration, foreign relations, transportation.

The feedback during the operation of a machine such as a computer or tool, for example the interaction between a driver and the position of his or her car on the road: by steering the driver influences this position, by observation this information returns to the driver.

In medicine, most medications can be safely used with other medicines, but particular combinations of medicines need to be monitored for interactions, often by the pharmacist. Interactions between medications fall generally into one of two main categories:

pharmacokinetic : Involving the absorption, distribution, metabolism, and excretion of one or both of the interacting drugs upon the other.

In terms of efficacy, there can be three types of interactions between medications: additive, synergistic, and antagonistic.

Additive interaction means the effect of two chemicals is equal to the sum of the effect of the two chemicals taken separately. This is usually due to the two chemicals acting on the body via same or similar mechanism. Examples are aspirin and motrin, alcohol and depressant, tranquilizer and painkiller.

Synergistic interaction means that the effect of two chemicals taken together is greater than the sum of their separate effect at the same doses. An example is pesticide and fertilizer; the biological effect is devastating.

Antagonistic interaction means that the effect of two chemicals is actually less than the sum of the effect of the two drugs taken independently of each other. This is because the second chemical increases the excretion of the first, or even directly blocks its toxic actions. Antagonism forms the basis for antidotes of poisonings.

Geneticists work with a number of different genetic interaction modes to characterize how the combination of two mutations affect (or does not affect) the phenotype:[1]
noninteractive, synthetic, asynthetic, suppressive, epistatic, conditional, additive, single-nonmonotonic and double-nonmonotonic.
Further characterizations is enhancement interaction and nonadditive interaction. Biosemioticists investigate sign-mediated interactions within and between organisms that underlie syntactic, pragmatic and semantic rules.

The word epistasis is also used for genetic interaction in some contexts.

In sociology, social interaction is a dynamic, changing sequence of social actions between individuals (or groups) who modify their actions and reactions due to the actions by their interaction partner(s). Social interactions can be differentiated into accidental, repeated, regular, and regulated. Social interactions form the basis of social relations.

If we were examining the effect of two variables, gender and premature birth, on health outcomes, we would describe any difference in health outcome scores between genders as a main effect. Similarly any difference in scores of full term/premature birth would be described as a main effect. The presence of an interaction effect implies that the effect of gender on health outcome varies as a function of premature birth status.

In media, interactivity is a feature of the media in question and as digital technology becomes more accessible to the masses interest in interactivity is increasing and becoming a cultural trend especially in the arts.